1. Field of the Invention
The invention relates to methods of design high-performance codes for the multiple-input-multiple-output MIMO wireless communication systems and the apparatus which use them.
2. Description of the Prior Art
For wireless communication systems, many codes have been devised to combat channel fading. However, many of these coding schemes were developed for single-transmit/receive-antenna systems, which cannot be directly applied to the recent multiple-input-multiple-output (MIMO) wireless systems. It is still an open problem to find good codes that can take full advantage of multiple transmit/receive antennas.
In the recent years, some concatenated codes were presented for open-loop MIMO systems. Several block codes have been designed based on channel mean/covariance information at the transmitter. Block code is an error detection and/or correction code in which the encoded block consists of N symbols, containing K information symbols (K<N) and N-K redundant check symbols, such that most naturally occurring errors can be detected and/or corrected.
More specifically, space-time trellis codes (STTCs) have been introduced to provide improved error performance for wireless systems using multiple transmit antennas. Space-time block codes operate on a block of input symbols producing a matrix output whose columns represent time and rows represent antennas. Unlike traditional single antenna block codes for the additive white Gaussian noise (AWGN) channel, most space-time block codes do not provide coding gain. Coding gain refers to the improvement in decibels (dB) that a particular code offers over other option. An improvement in coding gain can provide the designer with options such as reducing transmission power or bandwidth. Their key feature is the provision of full diversity with extremely low encoder/decoder complexity. Diversity is the property of being made up of two or more different elements, media, or methods. Diversity gain is the ratio of the signal field strength obtained by diversity combining to the signal strength obtained by a single path. Diversity gain is usually expressed in dB. In addition, they are optimal over all unitary codes with respect to the union bound on error probability. The best known codes for real constellations have been designed for a practical range of transmit antennas (2 to 8).
Space-time trellis codes operate on one input symbol at a time producing a sequence of vector symbols whose length represents antennas. Like traditional trellis coded modulation (TCM) for the single-antenna channel, space-time trellis codes provide coding gain. Since they also provide full diversity gain, their key advantage over space-time block codes is the provision of coding gain. Their disadvantage is that they are extremely difficult to design and require a computationally intensive encoder and decoder.
It was shown that, for an open-loop system, where only the receiver has full knowledge of the channel, the rank and determinant of the pair wise codeword difference matrix determine the coding gain of the corresponding space time trellis code. If the pair wise codeword difference matrix is full rank, full spatial diversity is obtained.
Recently, the original space time trellis codes in have been enhanced by the super-orthogonal space time trellis codes (SOSTTC), and the super-quasi-orthogonal space time trellis codes (SQOSTTC). In these new trellis codes, a standard multiple trellis coded modulation (M-TCM) encoder serves as the outer encoder, while the space-time block codes (STBCs), or the quasi-orthogonal space time block codes (QOSTBCs), are used as building blocks for the inner codes. Multiple trellis coded modulation (MTCM) is trellis code in which each trellis branch corresponds to multiple symbol transmissions from each transmit antenna. The super orthogonal space time trellis code and super quasi-orthogonal space time trellis code enjoy full spatial diversity, higher coding gain, as well as simple implementation.
The space-time coding schemes mentioned above do not exploit the channel knowledge at the transmitter. However, it is clear that with additional channel state information (CSI), the space-time transmission could be further improved.
STBC beam forming schemes have been proposed based on the channel mean or covariance feedback. The schemes use precoding matrices which are constructed based on imperfect feedback of the mean or covariance of a complex Gaussian channel. Nevertheless, these schemes use complicated eigen-analysis to construct the optimal precoding matrices. In addition, the resulting beam forming matrix accomplishes optimal power loading, thus it normally incurs a high peak to average power ratio (PAPR) at the transmitter, which significantly increases the complexity and cost of the system. It may be difficult to implement these beam forming schemes in practical digital communication systems.
Besides these STBC-based beam forming schemes, there are several other schemes that are based on traditional one-dimensional beam forming. Among them, a very promising scheme is the co-phase transmission (CPT) scheme. In cophase transmission, the relative channel phase information is uniformly quantized and sent back to the transmitter. On the transmit side, a rotation vector is applied to the transmission symbol. The rotation vector is constructed such that the signals from the different transmit antennas are added coherently at the receiver antenna, thus the receive signal to noise ratio (SNR) is maximized. The major advantage of cophase transmission is its easy implementation. With only a few feedback bits, significant performance improvement is attained. On the other hand, unlike the space time trellis code schemes, the original cophase transmission scheme does not provide any coding gain from the space-time transmission.
In the prior art, the channel state information has not been used to design concatenated codes for multiple-input multiple-output (MIMO) communication systems. Furthermore, the design criterions in the prior art are developed based on channel mean/covariance information. However, what is needed is a coding scheme for MIMO wireless communication which is not subject to the foregoing limitations of the prior art.